The present invention relates to an improved methodology of discriminating between desired and undesired readings or events measured by radiation detectors. Specifically, the method is intended for use in radiation detectors having non-Gaussian energy response functions.
Gamma ray cameras are relatively well known devices used to detect gamma ray emissions from radioactive decay. A known gamma ray camera described in U.S. Pat. No. 3,011,057 for RADIATION IMAGE DEVICE, hereby incorporated by reference, uses a single sodium iodide (xe2x80x9cNaIxe2x80x9d) scintillation detector crystal to detect gamma ray emissions. The detector crystal is positioned to receive a portion of the gamma ray emissions from the radioactive decay. When a gamma ray strikes and is absorbed in the detector crystal, the energy of the gamma ray is converted into a large number of scintillation photons that emanate from the point of the gamma ray""s absorption in the detector. A photomultiplier tube, optically coupled to the detector crystal, detects a fraction of these scintillation photons and produces an electronic signal that is proportional to the number of detected incident scintillation photons. The gamma ray camera typically has several photomultiplier tubes placed in different positions, with the signals from the different photomultiplier tubes being combined to provide an indication of the positions and energies of detected gamma rays.
Gamma ray cameras are frequently used in nuclear medical imaging to record pictures of a radiation field emanating from a subject""s body. The gamma rays originate from a decay of a radioactive tracer that has been introduced into the subject""s body. The radioactive tracer, such as 99mTc, is a pharmaceutical compound to which a gamma ray emitting nuclide has been attached and which undergoes some physiological process of interest after introduction into the body. For example, the tracer may accumulate in areas of high blood flow, thereby pinpointing areas of physiological activity.
Only substantially collimated gamma rays reach the gamma ray camera because a collimator usually is placed between the source of radiation and the scintillator. Of these collimated gamma rays, the gamma ray camera only detects the small fraction of gamma rays that impinge a detector, such as the above-described NaI detector crystal and photomultiplier tube assembly. From these detected gamma rays, the gamma ray camera typically selects a desired sample of gamma rays based upon their measured energy according to a process described in greater detail below. The gamma ray camera then records in an image memory the number of gamma events detected at each of a number of spatial locations corresponding to regions in the patient""s body from which the gamma rays emanated. The data in the image memory is then used to form an image that corresponds to the distribution of the detected gamma events, thereby providing an image of the organ, tissue or body region of interest.
The quality of the gamma ray camera""s image of the distribution of the radioactive tracer is dependent on the sensitivity of the detectors in the camera. As the sensitivity of the detector to gamma rays is increased, more gamma rays are detected and incorporated into the image, because the overall detection system sensitivity, combined with the activity of radioactive tracer presence in the body, determines the number of detected gamma ray events. Creating an image from a small set of gamma ray events may result in an inaccurate image because the small sample may not accurately represent the true distribution of the radioactive tracer within the subject""s body. In particular, a reduction in the number of detected events, or xe2x80x9ccounts,xe2x80x9d results in increased fractional statistical uncertainty because the emission and subsequent detection of gamma-rays are both random, stochastic processes. As a result, for any single gamma ray detector, the standard deviation in the number of counts in that detector increases, as a fraction or as a percentage, as the number of counts decreases. Therefore, the image would be more noisy in terms of image quality as a result of decreasing the total number of counts contributing to the image.
However, increasing the total number of counts by the inclusion of false events may degrade the image produced by the camera. One source of false events is the scattering of some of the gamma rays between the point of emission and exiting the patient""s body. These scattered gamma rays do not provide an accurate indication of the distribution of the radioactive tracer within the patient""s body because the scattering changes the gamma ray""s direction and the measured energy. As a result, the inclusion of too many scattered gamma rays in the image memory causes the gamma ray camera to produce an inaccurate image with poor contrast.
Gamma ray detectors, therefore, seek to strike a balance between scattered and unscattered gamma rays without a loss of sensitivity. Typically, this result is accomplished by evaluating the energy levels of the detected gamma ray events and selecting those gamma ray events that fall within a preset xe2x80x9cwindowxe2x80x9d or range of acceptable energy levels. This technique is based on the assumption that gamma rays of abnormally low or high energy levels have been altered prior to detection or did not originate from the radioactive tracer of interest and, accordingly, do not provide reliable indications of the rays"" direction of origination. The unscattered gamma ray events tend to fall within the window, whereas the scattered gamma ray events tend to fall outside the window. While a small number of scattered gamma rays would also fall within this energy window, these detected scattered gamma rays would be greatly outnumbered by the unscattered events. The gamma ray camera may vary the energy window as needed to achieve a desired result; for example, a narrower energy window may continue to accept a large number of unscattered gamma ray events while excluding more scattered events.
Frequently, the energy resolution of the gamma ray detector, such as the above-described NaI detector, can be characterized by a Gaussian distribution. The energy resolution of the gamma ray detector is then described by the full-width at half-maximum (xe2x80x9cFWHMxe2x80x9d) of the Gaussian shaped photopeak.
As explained above, gamma ray cameras generally use energy windows to discriminate between scattered gamma rays and gamma rays that reached the detector without interaction. The use of the energy window that defines a range of acceptable energy events is fairly effective in gamma ray cameras having Gaussian energy response functions because a large percentage of the desired gamma ray events are concentrated within a small range of energy levels. For instance, with a monoenergetic gamma ray emitting isotope in the absence of any scattered gamma rays, an energy window of approximately twice the width of the FWHM centered over the photopeak would select on the order of 99% of the total unscattered events.
However, the use of an energy window has the unavoidable disadvantage of excluding some of the desired gamma ray events while including some of the undesired gamma ray events, regardless of the width of the energy window. It is therefore the goal of the present invention to present an improved methodology to differentiate between the desired and undesired gamma ray events, without decreasing the sensitivity of the detector.
U.S. Pat. No. 5,561,297 for SCATTER CORRECTING GAMMA CAMERA, incorporated by reference herein in its entirety, discloses a method that improves the quality of an image by reducing the inclusion of scattered gamma rays into an image. The method measures the response of the imaging detector to substantially unscattered gamma rays, fitting a calibration function to the detector response to the unscattered gamma rays for each spatial location. The method then fits the response of the imaging detector to a field of both scattered and unscattered gamma rays with a combination of the measured responses to the unscattered gamma rays and an estimated shape for the spectrum of scattered gamma rays. The response function is a Gaussian function and the centroid energy and standard deviation of the Gaussian function are the spatially varying parameters that are determined by fitting the unscattered gamma ray spectrum for each spatial location.
The disadvantages of using an energy window are especially evident with gamma ray detectors having non-Gaussian energy response functions. In detectors with an energy response function that is not Gaussian shaped, a significant number of the desired gamma ray events occur away from the photopeak of the energy response function. The use of a narrow energy window would exclude these desired gamma ray events, whereas the use of a wide energy window to include these off-peak values would include too many of the undesired, scattered gamma ray events.
One type of gamma ray detector that has a non-Gaussian shaped energy response function is a solid state, pixelated detector made from the room temperature semiconductor material cadmium zinc telluride (xe2x80x9cCZTxe2x80x9d). For example, the measured energy response function for one CZT detector has a photopeak in the range of 5% FWHM and a significant tailing toward the lower energy levels. Therefore, a large number of the desired gamma ray events detected with this CZT detector occur away from the photopeak and would be wrongly excluded with the use of a narrow energy window.
FIG. 1 illustrates a typical energy spectrum of a single pixel of a pixelated CZT detector exposed to substantially unscattered 140 keV gamma rays. In FIG. 1, the peak centered about 140 keV represents the gamma rays that have been absorbed substantially within the center portion of the single pixel, and the distribution of signal amplitudes of these events is approximately Gaussian. However, a significant number of gamma rays are also detected in the portion of the energy response spectrum that tails toward the lower energies. This phenomenon is caused, in part, by gamma ray absorption events that do not confine all charge creation to within a single pixel and by non-ideal charge collection. Because the illustrated response function represents the distribution of measured signals from only a single pixel, charge that is lost from the pixel and shared with adjacent pixels is not included in the response function. As a result, gamma ray absorption events in which the charge collection is incomplete due to charge sharing with other pixels are lost from the peak region and contribute to the low energy tailing.
The use of a simple energy window to discriminate unscattered from scattered gamma-rays for detectors with non-Gaussian response functions, such as the CZT detector, is problematic. The use of a simple energy window for the detector pixel shown in FIG. 1 would reduce the overall sensitivity of the detector if the window width were made narrow (about two times the FWHM width of the peak). A narrow window would encompass the peak but not the unscattered events that contribute to the tailing toward lower energies, so a large number of the desired gamma ray events would be excluded, reducing the sensitivity of the detector. Conversely, using a very wide energy window to include the peak region and the tailing toward lower energies would also include a high fraction of scattered gamma rays in the image, and thus would degrade the image quality.
It is therefore a further goal of the present invention to provide a reliable methodology for estimating unscattered gamma rays detected by a gamma ray detector having a non-Gaussian energy response function.
These and other needs are addressed in the present invention through a novel method for discriminating between scattered and unscattered detected radiation events, so that the sensitivity of the radiation detector may be preserved without causing the inclusion of a large number of undesired radiation events. This method includes the steps of: (1) empirically determining the energy response function of the radiation detector; and (2) then using the empirically determined energy response function during the actual detection process to differentiate between desired and undesired radiation events.
In a preferred embodiment, the energy response function for each of the detectors is determined by simulating the condition for the subsequent, actual measurement. During this process, the detector is illuminated with a uniform field of radiation to be measured. The detector should be exposed to a large number of radiation photons, and the number of undesired rays is minimized. For example, the detector will be exposed to only direct, unscattered radiation. This simulation produces an energy response function for the detector under conditions that approximate the actual measurement.
During the actual measurement, the detector produces a second energy response spectrum that represents both the desired and undesired events. Then, a least squares estimate of the number of desired, unscattered events is produced by taking the dot product of (1) the spectrum acquired during actual measurement and (2) a weighting vector determined during the calibration step.